The present invention relates, in general, to semiconductor packages, and more particularly, to a plastic semiconductor package having a heat sink and stress relieving wall.
Semiconductor packages are manufactured in various shapes and sizes and typically have a number of interface leads protruding from one or more sides. Typically, a high power semiconductor die is mounted on a heat sink which makes up the bottom surface of the package. Since the plastic encapsulation does not completely surround the heat sink, delamination or separation of the plastic from the heat sink can occur. As a result, a locking device is typically provided in the heat sink to increase the adhesion of the plastic to the heat sink. Even though the plastic material is firmly fastened to the heat sink, the semiconductor die can crack or be separated from the heat sink due to the difference of coefficient of thermal expansion between the plastic material and the heat sink. Even if failure does not occur, the semiconductor die is placed under high stress, potentially altering the electrical characteristics of the die.
An example of a locking device is found in U.S. Pat. Pat. No. 4,107,727, entitled "A Resin Sealed Semiconductor Device", issued Aug. 15, 1978 to Ryuichi Ikezawa et al, which is incorporated herein by reference. A multi-piece structure is used which incorporates a flange serving to hold the plastic in intimate contact with the heat sink. The semiconductor die is mounted on a pedestal between the locking devices. One embodiment of the pedestal includes a shallow recess in which to bond the semiconductor die. Another form of locking device is shown in U.S. Pat. No. 4,994,897 issued Feb. 19, 1991, entitled "Multi-level Semiconductor Package" issued Feb, 19 1991 to T. R. Golubic et al, which is incorporated herein by reference. The plastic semiconductor package has a plurality of protrusions which serve to lock the plastic material to the heat sink. In addition, the semiconductor die is recessed slightly into the pedestal so as to minimize the length of wire bonds.
These packages address the problem of locking the plastic material to the heat sink. Some designs allow excess die bond material to extend beyond the intended area and flow into the recesses intended for locking of the plastic material. Furthermore no attempt is made to provide stress relief for the semiconductor die or the die bond. Accordingly even though the semiconductor die is recessed slightly in certain of these packages and others provide a flange for a locking device, the depth of recess or height of the flange is not considered to be sufficient to provide significant stress relief for the semiconductor die.
There is a need for a plastic package for housing a semiconductor die which provides stress relief for the semiconductor die. The package should be economical to build, extremely reliable, and should lock the plastic material to the heat sink. The method should be easy to assemble and die bond material extending beyond the intended area should not compromise reliability.